net: Check against nb->tail in grub_netbuff_pull()

net: Check against nb->tail in grub_netbuff_pull()

GRUB netbuff structure members track 2 different things: the extent of memory
allocated for the packet, and the extent of memory currently being worked on.

This works out in the structure as follows:

  nb->head: beginning of the allocation
  nb->data: beginning of the working data
  nb->tail: end of the working data
  nb->end:  end of the allocation

The head and end pointers are set in grub_netbuff_alloc() and do not change.
The data and tail pointers are initialised to point at start of the
allocation (that is, head == data == tail initially), and are then
manipulated by grub_netbuff_*() functions. Key functions are as follows:

  - grub_netbuff_put():     "put" more data into the packet - advance nb->tail
  - grub_netbuff_unput():   trim the tail of the packet - retract nb->tail
  - grub_netbuff_pull():    "consume" some packet data - advance nb->data
  - grub_netbuff_reserve(): reserve space for future headers - advance nb->data and nb->tail
  - grub_netbuff_push():    "un-consume" data to allow headers to be written - retract nb->data

Each of those functions does some form of error checking. For example,
grub_netbuff_put() does not allow nb->tail to exceed nb->end, and
grub_netbuff_push() does not allow nb->data to be before nb->head.

However, grub_netbuff_pull()'s error checking is a bit weird. It advances nb->data
and checks that it does not exceed nb->end. That allows you to get into the
situation where nb->data > nb->tail, which should not be.

Make grub_netbuff_pull() check against both nb->tail and nb->end. In theory just
checking against ->tail should be sufficient but the extra check should be
cheap and seems like good defensive practice.

Signed-off-by: Daniel Axtens <dja@axtens.net>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>